1. Field of the Invention
The present invention relates to a plasma processing apparatus and a plasma processing method which can be suitably used as, e.g., a plasma CVD apparatus useful to semiconductor devices such as an electrophotographic photosensitive device, an image input line sensor, an image pickup device, and a photovoltaic device, a sputtering apparatus for forming, e.g., insulating films and metal interconnecting lines as semiconductor devices and optical elements, and an etching apparatus for semiconductor devices and the like.
2. Related Background Art
In the fabrication of semiconductors and the like devices, various plasma processing methods and apparatuses are used in accordance with the intended uses. For example, apparatuses and methods using the characteristics of plasma are used in, e.g., the formation of oxide films, nitride films, and amorphous silicon semiconductor films using a plasma CVD process, the formation of metal interconnecting layers using a sputtering process, and the fine processing techniques using etching. Also, as demands on the film quality and the performance are increasing recently, various improvements are being examined. In particular, a plasma process using high frequency power is extensively used because of the advantages that 1) discharge is stable, and 2) process can be applied to insulating materials such as an oxide film and a nitride film.
As one example of a plasma CVD apparatus generally used in the formation of deposited films, FIG. 1 shows a plasma processing apparatus as a film formation apparatus for an amorphous silicon film (to be referred to as an a-Si film hereinafter) for a cylindrical electrophotographic photosensitive body.
In FIG. 1, the apparatus comprises a reaction vessel 201 which can be evacuated. This reaction vessel 201 is connected to an evacuating means 209 for evacuating the vessel and a gas supply means 210 for supplying a gas into the vessel.
In the reaction vessel 201, a cylindrical cathode electrode 202 electrically insulated from the reaction vessel 201 by an insulating material 211 is arranged. Additionally, a cylindrical film formation substrate 203 is arranged as a counter electrode inside the cathode electrode 202. The film formation substrate 203 is held by a substrate holder 204 having a rotating mechanism driven by a motor 212. A heater 205 is positioned in the internal space of the film formation substrate 203. The film formation substrate 203 can be heated to a predetermined temperature from the inside by the heater 205 arranged in the internal space. The cathode electrode 202 is connected to a high frequency power supply 207 for discharge via a matching circuit 208.
Note that the oscillation frequency of a discharge high frequency power supply used in a plasma process such as plasma CVD is commonly 13.56 MHz. The oscillation frequency of the high frequency power supply 207 is also 13.56 MHz.
An a-Si film formation method using this plasma processing apparatus will be described below.
First, the reaction vessel 201 is evacuated to a high vacuum by the evacuating means 209. Thereafter, the gas supply means 210 supplies a source gas such as silane gas, disilane gas, methane gas, or ethane gas and a doping gas such as diborane gas to maintain the pressure at several tens of millitorr to a few torr. Subsequently, the high frequency power supply 207 supplies high frequency power of 13.56 MHz to the cathode electrode 202 to generate a plasma between the cathode electrode 202 and the film formation substrate 203, thereby decomposing the source gas. Consequently, an a-Si film is deposited on the film formation substrate 203 heated to about 200.degree. C. to 350.degree. C. by the heater 205.
In the above apparatus using the 13.56-MHz high frequency power, even the maximum deposition rate at which a-Si films meeting the performance of a recent electrophotographic photosensitive body can be formed is at most about 6 (.mu.m/hour). If the deposition rate is further increased, it is sometimes impossible to obtain satisfactory characteristics as a photosensitive body. Generally, when an a-Si film is used as an electrophotographic photosensitive body, a film thickness of at least 20 to 30 .mu.m is necessary to obtain charging power. Accordingly, a long processing time is required to manufacture an electrophotographic photosensitive body meeting the required performance.
As a method of increasing the deposition rate, a plasma CVD process (Plasma Chemistry and Plasma Processing, Vol. 7, No. 3, (1987) pp. 267-273) is reported. This technique suggests that the deposition rate can be increased without lowering the performance of the deposited film by increasing the discharge frequency to be higher than 13.56 MHz by using a high frequency power supply with a frequency higher than 13.56 MHz. This method of increasing the discharge frequency is performed in the field of sputtering and the like and has been extensively studied.
As described above, the conventional plasma processing apparatus has the drawback that a long processing time is required to manufacture an electrophotographic photosensitive body and the like. Therefore, it is necessary to decrease the deposition time without lowering the performance of the deposited film.
Unfortunately, it is found that if a frequency higher than 13.36 MHz, e.g., a high frequency of 100 MHz, is directly applied to the conventional apparatus, the deposition rate of the deposited film varies, or etching cannot be evenly performed. That is, it turns out that it is difficult to perform uniform plasma processing or film deposition on a substrate with a relatively large area.
This degrades various characteristics of the deposited film. The conventional plasma processing apparatus has many problems to be solved in order to form a deposited film having uniform characteristics at a uniform deposition rate.